This invention relates to the process referred to as "bump bonding" or "flip chip bonding"; and it is directed to the problem of successfully aligning and bonding a multiplicity of closely-spaced conductive bumps formed on two facing surfaces, e.g., the bonding of a substrate carrying a two-dimensional array of photodetectors to a substrate which has circuitry providing individual leads to each detector. Also, there are numerous other uses for successful bump bonding techniques.
Such a structure requires effective bonding of many individual sets of conductive bumps. Because of the presence of photodetectors, which can be damaged by temperatures above 80.degree. C., the practice heretofore has been to bond such photodetector arrays solely by pressure (to cause the metal to cold flow), avoiding the use of heat to assist in the bonding process. It should be noted here that indium's melting point is 157.4.degree. C.
The generally favored material used in the bonding bumps is indium. It is an effective electrical conductor; and it is relatively soft, permitting individual bumps to be "squashed" (cold flow) under pressure. This deformability of the bumps is necessary, not only to cause a mechanical bond between bumps, but also, because the opposite planar surfaces carrying the aligned bumps can only be held to a flatness tolerance of about two or three microns. In order to permit substantially all of the bump sets to engage, those which engage first must be soft enough to compensate for the lack of absolute flatness of the planar surfaces. Also, because of its softness, indium minimizes the problems caused by mismatch of thermal coefficients of expansion of the detector substrate and the substrate which has the circuitry.
Indium as the bonding material does, however, have certain deficiencies, due primarily to its tendency to oxidize readily. Indium oxide is an insulator, which tends to defeat the purpose of the bond. Also, indium oxide tends to weaken the mechanical bond between the aligned bumps. A layer of gold on top of the indium bumps has been tried by some experiments, as a means of solving the oxidation problem. However, that has failed to solve the problem because the indium tends to diffuse through the gold, so that the indium still reaches the top of the bump and oxidizes.
In some uses of flip-chip bonding methods, i.e., bonding semiconductor chips, these problems were avoided by using bonding materials other than indium, and by using relatively high temperatures to cause such materials to flow. A solder reflow process using silver or gold bumps is popular in the industry. As stated above, when photo-detectors are involved, high temperatures cannot be used safely.